Defender: Vaporeon vs Blastoise vs Gyrados

Vaporeon is a lot better defender than Blastoise. You could have just used the defender tier list on this site (https://pokemongo.gamepress.gg/gym-defenders-tier-list) to figure that out.

Unfortuantly, in Pokemon GO (and even the main series games) Vaporeon greatly outclasses Blastoise as a pure water type. It has access to the good defense move Water Pulse, insane HP and high overall CP backing it up. However, if you manage to get a Blastoise with Ice beam, it can give an attacking Exeggutor/Venusaur/Vileplume/Victreebell an unpleasant surprise. Gyarados suffers from the same issues as Blastoise, but even worse since he does even less damage with Bite and is very weak to Electric.